Help! Why is my 12 week old baby suddenly changing his sleep patterns day and night?!

My FF baby is 12 weeks on tuesday and has slept for 5-6 hours at night 8pm -1/2am) for the last month, he then would wake between 5-7am (for the last month I've been trying to feed at 7am giving him a snack if he wakes and asks for it before then!) he was feeding every 3 hours 4oz then we upped it to 5 suggested by the hv but this last 2weeks become very distracted in the day, not taking the full 5oz so the last 3 days I have waited 4 hours, (he doesn't mind the wait, 13lb 2 from 6lb 7 at birth so growing well!!) 4 nights ago he slept 8pm - 5am! (We have a strict bedtime routine: part feed, bath, massage, finish feed 7/6oz.) since then he has been waking up more in the night, I try holding him to sleep as this used to work, gently swaying or placing my hand on him But it's been working Less. I will feed him if he do any settle after trying this and a nappy change. Day naps have gone to 30-40 mins too! Growth spurt? Transition time? Help 1st baby